A diptych is a sort of notebook, formed by the union of two tablets, placed one upon the other and united by rings or by a hinge. These tablets were made of wood, ivory, bone. or metal. Their inner surfaces had ordinarily a raised frame and were covered with wax, upon which characters were scratched by means of a stylus. Diptychs were known among the Greeks from the sixth century before Christ. They served as copy-books for the exercise of penmanship, for correspondence, and various other uses. The Roman military certificates, privilegia militum, were a kind of diptych. Between the two tablets others were sometimes inserted and the diptych would then be called a triptych, polyptych, etc. The term diptych is often restricted to a highly ornamented type of notebooks. They were generally made out of ivory with carved work, and were sometimes from twelve to sixteen inches in height. In the fourth and fifth centuries a distinction arose between profane and ecclesiastical (liturgical) diptychs, the former being frequently given as presents by high-placed persons. It was customary to commemorate in this way one's elevation to a public office, or any event of personal importance, e.g. a marriage. The consuls, on the day of the installation, were wont to offer diptychs to their friends and even to the emperor. Those presented to the latter often had a border of gold and were quite large. Their tablets often exhibited on a central plate the portrait of the sovereign, surrounded by four other plates. The (undated) Barberini ivory at the Louvre is thus constructed and once served as an ecclesiastical diptych (see below). Some believe it to be the binding of a books offered to the emperor. Strzygowski holds it to be of Egyptian origin and thinks that the portrait is that of Constantine the Great, defender of the Faith. The oldest dated consular diptych is that of Probus (406); it is kept in the treasury of the cathedral of Aosta, Piedmont. The latest is that of the Eastern consul, Basilius (541), one tablet of which is at the Uffizi Museum in Florence and the other at the Brera in Milan. The Theodosian Code (384) forbade the offering of ivory diptychs to any but the regular (i.e. not honorary) consuls. The tablet at the Mayer Museum in Liverpool, bearing the image of Marcus Aurelius (d. l80), is prior to this enactment. The consular diptychs are recognizable by their inscriptions or by the figure of the consul which they bear. On the diptych of Boetius at Brescia (487) and several others of the same type the consul is clad in a trabea (a kind of toga); he holds in his left hand the scipio (consular sceptre) and in his right the mappa circensis, or white cloth which he used to wave as the signal for the games in the circus. These games (ludi) or other liberalities offered to the people by the consul were frequently represented on the tablets of the diptychs.

There is less certainty concerning the diptychs of officials other than consuls, e.g. praetors, quaestors, etc. The diptych of Rufius Probianus V.C. (i.e. vir clarissimus) vicarius urbis Romae, in the Berlin Museum, is the most precious relic of this class, and probably dates from the end of the fourth century. Among the diptychs of private individuals that of Gallienus Concessus, discovered at Rome on the Esquiline, exhibits only the name of its owner. Others were richly ornamented and reproduced often some of the masterpieces of ancient art. Thus on a diptych in the Mayer Museum, Liverpool, are seen Aesculapius and Telesphorus Hygieia, and Amor. The most beautiful of the profane diptychs was carved at the time of a marriage between the Symmachi and the Nicomachi (392 to 394, or 401). It represents on each leaf (one of which is at the South Kensington Museum and the other, in a very damaged condition, at Cluny) a woman performing a sacrifice. Many of the profane diptychs were preserved in the treasuries of the churches, where they were eventually used for liturgical purposes or enshrined in bookbindings or in goldsmith work. The diptych of Boetius, among others bears on the interior, some liturgical texts and religious paintings, attributed to the seventh century. The Liege diptych of the consul Anastasius (517), one leaf of which is at Berlin and the other at South Kensington, bears an inscription of forty-two lines and the prayer Communicantes from the Canon of the Mass. Another of the same consul (in the BibliothèqueNationale, Paris) has a list of the bishops of Bourges. At the cathedral of Monza, Lombardy, a diptych represents in the dress of consuls king David and St. Gregory the Great. It is perhaps an ancient consular diptych, transformed in the eighth or ninth century; according to some it appears to be of ecclesiastical origin. Many carved diptychs reproduced purely religious subjects. On a diptych in the treasury of Rouen cathedral the figure of St. Paul is exactly the same as that on a sarcophagus in Gaul. A diptych leaf in the treasury of Tongres was evidently influenced by the carvings on the cathedra of St. Maximinus at Ravenna, and seems to have belonged to an ancient episcopal see. Certain diptychs with religious subjects, e.g. the Holy Sepulchre and the holy women at the Tomb of Christ (Milan), an angel (British Museum), probably date from the fourth or fifth century. Diptych leaves divided into five compartments have generally served as a cover for copies of the Gospels. The diptychs, though often clumsily executed, are important for the history of sculpture, there being a good number of them extant, and several being accurately dated. At different periods in the Middle Ages numerous diptychs or triptychs of ivory were made, to serve as little devotional panels.

The liturgical use of diptychs offers considerable interest. In the early Christian ages it was customary to write on diptychs the names of those, living or dead, who were considered as members of the Church a signal evidence of the doctrine of the Communion of Saints. Hence the terms "diptychs of the living" and "diptychs of the dead." Such liturgical diptychs varied in shape and dimension. Their use (sacrae tabulae, matriculae, libri vivorum et mortuorum) is attested in the writings of St. Cyprian (third century) and by the history of St. John Chrysostom (fourth century), nor did they disappear from the churches until the twelfth century in the West and the fourteenth century in the East. In the ecclesiastical life of antiquity these liturgical diptychs served various purposes. It is probable that the names of the baptized were written on diptychs, which were thus a kind of baptismal register. The "diptychs of the living" would include the names of the pope, bishops, and illustrious persons, both lay and ecclesiastical, of the benefactors of a church, and of those who offered the Holy Sacrifice. To these names were sometimes added those of the Blessed Virgin, of martyrs, and of other saints. From such diptychs came the first ecclesiastical calendars and the martyrologies. The "diptychs of the dead" would include the names of persons otherwise qualified for inscription on the diptychs of the living, e.g. the bishops of the community (also other bishops), moreover priests and laymen who had died in the odour of sanctity. It is to this kind of diptychs that the later necrologies owe their origin. Occasionally special diptychs were made to contain only the names of a series of bishops; in this way arose at an early date the episcopal lists or catalogues of occupants of sees. Whatever their immediate purpose the liturgical diptychs admitted only the names of persons in communion with the Church; the names of heretics and of excommunicated members were never inserted. Exclusion from these lists was a grave ecclesiastical penalty; the highest dignity, episcopal or imperial, would not avail to save the offender from its infliction. The content of the diptychs was read out, either from the ambo (q. v.) or from the altar by a priest or a deacon. In this respect a variety of customs obtained in different churches and at different periods, sometimes the diptychs were simply laid on the altar during Mass, and when read publicly, such reading did not always occur at the same stage of the Mass. The order of which traces are now seen in the Roman Canon of the Mass was the fixed usage of the Roman Church as early as the fifth century. In that venerable document a long passage after the Sanctus corresponding to the ancient recitation of the diptychs of the living; it contains, as is well known, mention of those for whom the Mass is offered, of the pope, of the bishop of the diocese, of the Blessed Virgin, and of several saints. At Easter and at Pentecost the Hanc igitur furnished a proper occasion to mention the names of the newly baptized, now mentioned only as a body. Finally the recitation of the "diptychs of the dead" is still recalled by the Memento which for the consecration.